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‘Nano-lightning’ could cool computer chips

By Celeste Biever

Jumping electric charges could waft breezes of ionised air through microchips, replacing the bulky, noisy fans that cool down today’s computers.

Researchers at Purdue University in West Lafayette, Indiana say their patent-pending technology could be built directly into a computer chip’s heat sink to provide a faster, quieter and lighter cooling system than the alternatives.

In current designs, a metallic sink absorbs the heat generated by currents in the microchip and is cooled by mechanical fans. But as engineers squeeze more functionality out of smaller chips, they are finding that the fans cannot cool down the chips fast enough or are too big for the device. “Heat is now a major factor limiting the size of laptops,” says Dan Schlitz of Purdue University.

So researchers have come up with a range of alternatives including piezo-electric fans and tiny, cold-water pumps. But there is always a trade-off&colon; air is preferable to water, because it is does not need to be encased, but water is attractive because it absorbs and releases heat more quickly.

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The Purdue technology is the first air-based system to produce a cooling rate similar to water – 40 watts per square centimetre. “That is why we are excited,” says Suresh Garimella, who led the research and is engaged in seven other chip-cooling projects.

Air cushion

The secret is producing the air-flow right at the wall of the heat sink. Blowing it down from above using a fan creates a cushion of stationary air that impedes heat dissipation. But the new system consists of 300 electrodes that ionise and then pump the air molecules across the surface.

On one side of the device are the negatively charged electrodes, bristling with long, slender carbon nanotubes to concentrate the electric field. When the voltage is switched on, electrons jump the 10 microns from the negative to positive electrodes.

This knocks electrons off the air molecules to produce a cloud of positively charged ions. While the phenomenon is similar to lightning, it occurs at much lower voltages and no actual sparks are produced.

The positive ion cloud then drifts towards a negative electrode, but before it gets there, the voltage is switched to another electrode in a different position. In this way the positively charged cloud is pulled across the surface, carrying heat as it goes. Cycling the voltages creates a continuous flow.

Conversion rate

The efficiency with which the voltage is converted into electrons is as yet unknown. But any leftover voltage will be converted into heat, which hampers the cooling effect.

“This kind of electrically driven flow traditionally has low rates of conversion,” admits Garimella. However the device has successfully cooled warm metal surfaces, although it has not yet been integrated with a computer chip.

“This is promising research, but there are a range of competing technologies,” notes Richard Smith, who studies heat dissipation in electronic systems at the US National Science Foundation, which funded the research.